Switching power converters are widely used in computers, telecommunication systems, office automation equipment and electrical systems. Power conversion in switching power converters is usually done by switched mode pulse width modulating (PWM) circuits which typically include a switch controllable by a constant switching frequency and the output power of the power converter is controlled by varying the duty cycle of the switching signal. Such power supply devices are commonly known as pulse width modulation power converters.
In order to provide high power density, it is desirable that PWM power converters operate at high frequencies. However, because of inherent component characteristics such as diode stored charge, diode reverse recovery and device switching losses, the switching frequency of conventional power converters of up to 5 kW rating are usually below 200 kHz. With the application of lossless snubber networks such as zero voltage/current switching circuits to alleviate these problems, switching power converters now commonly operate between 500 kHz to 1 MHz.
Rectifying diodes are indispensable in conventional switching power converters. However, in addition to being a major source of electromagnetic interference (EMI) or radio-frequency interference (RFI), the reverse recovery of a rectifying diode is also a major source of stress to the switching device of a switching power converter. This is because when the controllable frequency switch is switched on (closed), it will cause a conducting diode to cease to conduct. At this point, the current through the diode decreases until the diode no longer conducts. Any charge remaining on the diode is then removed via the closed controllable switch. As the charge is being removed from the diode through the switch, the current flowing through the switch begins to rise and the current includes that from the power source, which is usually a voltage source, and that from the diode. This combined current can be more than twice the current from the voltage source alone. The high combined current, high rates of voltage and current changes when the switch is usually at a potential relatively low compared to the voltage source with one end of the controllable switch usually connected to the low potential point of the voltage source, generates significant EMI or RFI and creates considerable stresses in the switch.
Device switching losses are due to the inherent component characteristics of real switches. For real switching devices, the transitions from "open" to "close" and "close" to "open" are not instantaneous. This introduces a switch power loss represented by P=i.sub.switch .times.v.sub.switch.
To overcome these problems, reactive snubber networks have been devised to permit "soft-switching" in the process of power conversion. This means that the controlled switching occurs when there is little or no voltage across the controllable switch and/or when there is little or no current flowing through the controllable switch.
Power converters incorporating reactive snubber networks can be broadly categorized as "zero-voltage-switching" (ZVS) converters in which opening (switch off) and closing (switch on) of the switch occurs with little or no voltage across the switch or "zero-current-switching" (ZCS) converters in which opening (switch off) and closing (switch on) of the switch occurs with little or no current in the switch. Generally, ZCS snubbing is achieved by a purely inductive snubber network while the more commonly known ZVS topology is usually by way of a purely capacitive snubber having an antiparallel diode which is made to conduct shortly before the switch is closed (switch on). An example of this type of ZVS snubber network is described in U.S. Pat. No. 5,351,179 by Tsai, et. al.
A known disadvantage of this commonly known ZVS snubber topology is the requirement of additional circuitry to discharge the capacitor or a resistor to dissipate the energy stored in the capacitor during a part of the switching cycle. In such a case, power dissipation is merely shifted from switching losses to resistor losses. Furthermore, it is also well known that large current spikes may result if the snubber network is not accurately designed.
FIG. 1 is an example of a power converter with a dissipative snubber network. Referring to FIG. 1, when the controllable switch S1 is switched on, the current in the windings W1 of the transformer T1 increases and energy is stored in the transformer. When the controllable switch S1 is switched off (open), the energy stored in the transformer T1 will be released to the output capacitor through the secondary windings W2 of the transformer and the diode D2. The leakage energy stored in the transformer T1 can also be transferred to the capacitor C1 through the windings W1 and diode D1. In order to limit the possible peak voltage across the capacitor C1, a resistor is added in parallel to C1 to dissipate the leakage energy.
To alleviate the problem of power dissipation in the aforementioned dissipative snubber network, lossless snubber networks have been designed for use in soft switching power converters. Lossless snubber network are typically reset by inherent circuit operation and the energy stored in the snubber network during one part of the operating cycle is released for re-circulation during the other part of the cycle. An example of this "lossless" snubber is described in the background section and shown in FIG. 2 of U.S. Pat. No. 5,636,114 in which a switchable resonant circuit is formed across the controllable switch. The switchable resonant circuit is only electrically connected across the controllable switch near the end of the time when the controllable switch is off (open). Because of resonance, the voltage across the controllable switch is reduced to zero at the time when the controllable switch is to be switched on (closed). However, an extra active element and supporting circuitry is required to form the switchable resonant circuit which means increased circuit size, loss and costs.
FIG. 2 of this specification shows another example of a lossless snubber network that is described in U.S. Pat. No. 5,636,114 to Bhagwat et al. The snubber network in this topology comprises three series circuits: a first series circuit comprising the diode D21 and the capacitor C21, a second series circuit comprising a saturable inductor L22, the diode D24 and the Capacitor C22, and a third series circuit comprising the diode D23, inductor L21 and the diode D22. In this configuration, the capacitance of C21 must be large enough to completely discharge C22 in the inherent circuit operation. The operation of this network will be briefly explained below.
When the controllable switch S21 is first switched off, the winding energy and the leakage energy between the windings W1 and W2 will charge up the capacitor C21 with the potential at point A being higher than that at point B. At the same time the capacitor C22 is slowly charged up through the saturable inductor L22 and D24. After a short while, L22 is saturated and the voltage across C22, V.sub.C22, becomes equal to V.sub.C21 +Vin with the potential at point C higher than that at point D.
When the controllable switch S21 is next switched on (closed), point B of C21 goes to negative and the capacitor C22 is discharged to one diode drop below ground through the inductor L21, the diode D22 and the capacitor C21. The voltage across C22 will remain substantially unchanged for the remaining period of this switch-on cycle and the voltage across C21 is thus also lowered by the same amount.
When the controllable switch S21 is next switched off (open), the capacitor C21 will be charged to the same voltage level as when the switch S21 was first opened before and the voltage across the switch S21 will be equal to one diode drop (V.sub.D24) above point C of the capacitor C22. At this point, the voltage of the switch S21 will rise from zero and zero-voltage-switching is thereby achieved.